The invention relates to an optical transmitter system and more especially to an optical transmitter system for use in a wavelength division multiplex (WDM) optical communications system.
In conventional optical communication systems, communications traffic is communicated between a transmitter and receiver by means of optical radiation which is modulated with the communications traffic and which is conveyed by optical waveguiding means, typically an optical fibre. Optical radiation in the context of this patent application is defined as electromagnetic radiation within a free-space wavelength range from 560 nm to 2000 nm, although a free-space wavelength of substantially 1550 nm is a preferred part of this range.
In operation of optical communications system an optical fibre can suffer mechanical damage including breakage and it is desirable to prevent personnel in the vicinity of such a break being exposed to the escape of optical radiation, which may be of a level, which is hazardous to their eyesight or skin.
Two approaches to minimising this risk are: (i) to ensure, by design of the system, that the power level of the optical radiation carried by the fibre is always maintained below a “safe” level or (ii) to stop the supply of optical radiation to the fibre when there is an indication that a break may have occurred in the fibre. In the case of the latter the loss of the receipt of optical radiation at the receiver is taken as an indication of a potential fibre break. The latter method is often termed automatic laser shutdown (ALS). To re-start operation of the communications system, after ALS, it is known to re-start the laser after a specified period of time, currently 100 seconds, and pulse it for a short period of time. The duration of the pulse (typically two seconds) is selected to be sufficiently short as to not cause injury. When the receiver detects such a pulse it communicates back to the transmitter to re-commence normal operation.
EP 1003300 discloses a method of automatic restart of optical transmitters in an optical fibre communication system following the automatic shutdown of the optical transmitters as a consequence of a fault in the system. The method comprises periodically sending over the system, by means of a transmitter at the near end, a restart pulse and then detecting at the near end, within a given time interval, the return of the restart pulse which has been retransmitted (generated) by a transmitter at the far end. The method is characterised by successively increasing the duration of the restart pulse generated whenever the restart pulse is received/retransmitted. If a fault has actually been repaired, then, starting from a short duration restart pulse, in a short period of time, a longer retransmitted pulse is received at the near end before the transmitter at the near end generates a new restart pulse. In such a circumstance, which is indicative of the fault having been repaired, the transmitter is switched back on to continuous operation. In contrast, under the circumstance in which a fault has not been repaired, no retransmitted pulse will have been received at the near end when a new restart pulse is to be transmitted. The duration of the restart pulse will therefore be increased only once ensuring it remains within safety limits.
To increase the transmission capacity of the communications system it is current practice to employ wavelength division multiplexing in which the modulated optical radiation comprises a plurality of radiation components, termed wavelength channels, having mutually different wavelength bands. Each wavelength channel is modulated with respective communications traffic and all of the channels simultaneously conveyed over a single optical fibre. Systems with eight or more wavelength channels are often termed dense wavelength division multiplex (DWDM) systems.
For WDM systems the optical power conveyed by a fibre is the sum of the separate component (wavelength channel) power levels and it is undesirable to limit this total power level to a safe level since this would unduly limit the span of the optical link between the transmitter and receiver. This is especially so for DWDM systems operating with many tens of wavelength channels and such systems consequently favour ALS arrangements.
In active WDM systems one of the wavelength channels, termed an Optical Supervisory Channel (OSC), is reserved for controlling the communications system by passing control protocols between nodes. The loss of OSC at the receiver is used as an indication of a break in a fibre and the lasers for all the wavelength channels are accordingly shut down. To re-activate the system OSC only is pulsed at 100-second intervals for 2 seconds to test the optical fibre link. If the OSC reaches the receiver the lasers for the communications channels are accordingly reactivated.
A problem arises however in passive WDM systems in which the wavelength channels are passively combined and each originates from optical sources that operate in complete independence of one another. In passive systems there is no central control function for safely re-activating the sources and consequently it is preferred in such systems to limit the total optical power to a “safe” level.